1. Field of the Invention
The present invention relates in general to a wheel acceleration slip control apparatus in a vehicle brake system, and more particularly to an apparatus for applying brake to the drive wheels of a vehicle for preventing excessive slipping of the drive wheels during acceleration thereof.
2. Discussion of the Prior Art
When the drive wheels of a vehicle are accelerated (upon starting of the vehicle, for example), the drive wheels may slip on the road surface. To avoid excessive slipping of the drive wheels during acceleration thereof (hereinafter referred to as "acceleration slip" or "wheel acceleration slip", when appropriate), the brake system of some vehicles is provided with a brake control apparatus for applying brake to the drive wheels when an excessive slipping condition of the drive wheel or wheels is detected during acceleration of the drive wheels. An example of such a wheel acceleration slip control apparatus is disclosed in laid-open Publication No. 61-24655 of unexamined Japanese Patent Application. In this wheel acceleration slip control apparatus, brake cylinders for the right and left drive wheels are controlled independently of each other. When brake is applied to one of the two drive wheels upon detection of an excessive slipping condition of that one drive wheel, the rotating speed of the other drive wheel rises due to the function of the differential gear of the vehicle. As a result, this other drive wheel having a low slipping tendency tends to be in an excessively slipping condition. If brake is applied to this excessively slipping drive wheel, the amount of slip of the above-indicated one drive wheel tends to increase again. Thus, the brake applications to the one or other drive wheel so as to reduce or eliminate the slipping condition of the drive wheels may cause a slipping condition of the drive wheel which has not been braked. This phenomenon associated with the wheel acceleration slip will be referred to as "acceleration slip control interference" between the right and left drive wheels, when appropriate. The acceleration slip control interference may deteriorate the accuracy of control of the drive wheel brakes.
Another laid-open Publication No. 60-56662 of unexamined Japanese Patent Application discloses a wheel acceleration slip control apparatus, in which upon detection of an excessive slipping condition of one of the two drive wheels, brakes are applied to both of the drive wheels. While the brake applications to the two drive wheels may avoid the acceleration slip control interference indicated above, the drive force of the drive wheel having a comparatively low slipping tendency (hereinafter referred to as "low slipping tendency drive wheel" when appropriate) is unnecessarily restrained or limited, whereby the vehicle acceleration is unnecessarily lowered due to the brake application to the low slipping tendency drive wheel.
In the light of the above drawbacks of the known wheel acceleration slip control apparatus, the assignee of the present application proposed an improved wheel acceleration slip control apparatus as disclosed in laid-open Publication No. 64-44368 of unexamined Japanese Patent Application (published on Feb. 16, 1989), in an attempt to avoid both the acceleration slip control interference between the two drive wheels and the unnecessary reduction in the vehicle acceleration. This wheel acceleration slip control apparatus includes (a) a right and a left wheel brake for applying brake to the right and left drive wheels, independently of each other, (b) slip detecting means for detecting slipping conditions of the drive wheels upon acceleration of the drive wheels, (c) target determining means for determining control targets for controlling the right and left wheel brakes, based on the slipping conditions of the slip detecting means, and (d) brake control means for operating the right and left wheel brakes according to the control targets determined by the target determining means, so as to reduce the degree of slip of the drive wheels. The target determining means determines the control targets for the two drive wheels, which correspond to the detected slipping conditions of the drive wheels. The thus determined control target for the low slipping tendency drive wheel is compensated or adjusted so that the final control target to be eventually used for controlling the corresponding wheel brake is closer to the determined control target for the other drive wheel (which has a comparatively large slipping tendency). Namely, the nominal control target for the low slipping tendency drive wheel is compensated by a predetermined amount so that the difference between the final control targets for the two drive wheels is reduced as compared with that prior to the compensation. This arrangement may avoid the acceleration slip control interference, while preventing unnecessary lowering of the vehicle acceleration due to reduction of the drive force of the low slipping tendency drive wheel.
However, the above proposed wheel acceleration slip control apparatus still suffers from a problem which is experienced under certain situations. That is, the solution provided by the proposed apparatus is not sufficient in eliminating both of the two conventionally encountered drawbacks, i.e., the lowering of the vehicle acceleration, and the acceleration slip control interference between the two drive wheels.
For instance, the amount of adjustment or compensation of the nominal control target for the low slipping tendency drive wheel is set to be comparatively large, if it is assumed that the coefficient of friction of the road surface is relatively low and is even for the right and left drive wheels. In one exemplary form of the wheel acceleration slip control apparatus, the pressure in each of the wheel cylinders for the right and left drive wheels is controlled in a selected one of five different pressure control modes, i.e., a fast increase mode, a slow increase mode, a pressure hold mode, a slow decrease mode and a fast decrease mode. These control modes are consecutively numbered in the order of description, that is, in the order of the magnitude of an effect of reducing the degree of slip of the drive wheels. In this case, for example, the nominal pressure control mode corresponding to the detected slipping condition of the low slipping tendency drive wheel may be replaced by a final pressure control mode which is two steps closer to the nominal and final pressure control mode of the other drive wheel, so that the difference between the numbers of the final pressure control modes is comparatively small. This arrangement is suitable because the two drive wheels should not have a large difference in the drive force since the friction coefficient of the road surface is even for the two drive wheels. Further, the above arrangement which uses the final control targets relatively close to each other is desirable because the drive wheels tend to easily suffer from the acceleration slip control interference since the friction coefficient of the road surface is relatively low. In this case, however, the vehicle acceleration is deteriorated when the vehicle actually runs on a road surface which has considerably different friction coefficients in right and left areas corresponding to the right and left drive wheels. Suppose the nominal pressure control modes for the two drive wheels are the "fast increase" and "slow decrease" modes, respectively, the final pressure control modes eventually used for the drive wheels are the "fast increase" and "slow increase" modes, respectively. In other words, the pressure in the wheel cylinder for the low slipping tendency drive wheel is controlled in the slow increase mode. This unnecessarily limits the drive force of the low slipping tendency drive wheel running on the area of the road surface which has the comparatively high friction coefficient. Accordingly, the vehicle acceleration is lowered. Further, the acceleration slip control interference will not easily take place on the road surface having different friction coefficients in the right and left areas, since the drive wheel running on the road surface area having the comparatively high friction coefficient will not easily slip even if the drive forces of the two drive wheels are considerably different from each other. In other words, it is not required that the final control target or pressure control mode for the less slipping tendency drive wheel be relatively close to that for the comparatively seriously slipping drive wheel. As explained above, the drive force of the low slipping tendency wheel and the vehicle acceleration are unnecessarily limited or reduced, according to the above arrangement wherein the amount of adjustment or compensation of the nominal control target to determine the final control target for the low slipping tendency drive wheel is set to be relatively large, assuming that the friction coefficient of the road surface is relatively low and is even for the right and left drive wheels.
If the amount of compensation to determine the final control target for the low slipping tendency drive wheel is set to be relatively small, in view of the road surface having different friction coefficients, the difference between the final control targets for the right and left drive wheels tends to be excessively large and the acceleration slip control interference tends to easily occur, where the vehicle runs on a road surface having a relatively low and even friction coefficient.